1. Field of the Invention
The present invention relates to an image processing method and apparatus for recording, for example, half-tone images.
2. Related Background Art
As image processing apparatuses for printing an image generated by a computer on a recording medium, image processing apparatuses using electrophotography such as laser beam printers are widely used nowadays. A variety of merits of these image processing apparatuses, such as high quality print, silence, and high speed, have caused to rapidly expand the fields of desk top publishing (DTP).
It is now common to further improve the image quality by incorporating high image quality technique such as a smoothing process for smoothing the detected edges of characters and figures. Some printer engines of a print mechanism have a resolution as high as 480 dpi (dot/inch) or 600 dpi, as different from the past standard of 240 dpi or 300 dpi. A combination of such a high resolution and a smoothing process has improved the print quality much better than a conventional quality.
As a method of printing an image having gradation levels, several methods have been used conventionally, including a dither method, a density pattern method, an error diffusion method, and the like. All these methods are a so-called quasi half-tone method by which a tonal level is expressed by a ratio of black dots to white dots in a predetermined area. Since a resolution in the main scan direction can be relatively easily changed especially for laser beam printers, a pulse width modulation method has been used for expressing a gradation, for example, by changing a width of a drive pulse of a laser diode in accordance with the level of image data. As compared with the quasi half-tone method, typically a dither method, the pulse width modulation method is excellent in that both the gradation and resolution can be satisfied at high level.
However, for pulse width modulation, each pixel is required to have multivalue data. For example, if an image of 256 tonal levels with 600 dpi is to be output, each pixel has 8-bit data so that a memory capacity becomes an eightfold of a conventional binary value print. For example, for an output of an A4 size image, an image memory of about 32 MB is required, considerably raising the cost.
As a means for solving the above problem, it can be considered that a data resolution is lowered. A resolution necessary for printing an image picture having a gradation like a photograph, is 200 lines/inch ("lines/inch" is approximately equivalent to "dpi") even for reprophotography. Therefore, a resolution of 150 to 300 dpi can provide a sufficiently high image quality through electrophotography. For an expression by 8 bits per one pixel with a resolution of 300 dpi, the image memory capacity for A4 size is 8 MB and a rise of cost can be suppressed greatly, although this capacity is larger than an expression by a binary value with a resolution of 600 dpi.
With this method, however, an output image is a so-called binary value picture, i.e., vector-developed character and figure data also has a resolution of 300 dpi, lowering the print quality.
As another means for solving the above problem, it can be considered that two types of memories for a multivalue picture and a binary picture are divisionally used. For example, data of a binary picture, i.e., vector-developed character and figure, is stored in a binary, 600 dpi memory, whereas multivalue data of an image picture is stored in an 8-bit, 150 dpi memory, and these data are synthesized when the image data is transferred to a printer engine. With this method, a total memory capacity necessary for A4 size print is 4 MB for the binary, 600 dpi memory and 2 MB for the 8-bit, 150 dpi memory, totaling 6 MB.
With this method, however, since two memories are used for a binary picture and a multivalue image picture, the memory control becomes complicated and a load on a CPU increases degrading the apparatus performance. Furthermore, in order to realize this method, present software and hardware architectures are required to be changed to a large degree. Since the hardware scale increases correspondingly, the cost is raised. Still further, there arises a problem of a boundary process when data with different resolutions are synthesized.